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Self-Stable WP/C Support with Excellent Co-Catalytic Functionality for Pt: Enhanced Catalytic Activity and Durability for Methanol Electro-Oxidation Yaqiang Duan, Ye Sun, Siyu Pan, Ying Dai, Liang Hao, and Jinlong Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09756 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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ACS Applied Materials & Interfaces

Self-Stable WP/C Support with Excellent Co-Catalytic Functionality for Pt: Enhanced

Catalytic

Activity

and

Durability

for

Methanol

Electro-Oxidation Yaqiang Duana,b, Ye Suna,b, Siyu Pana,b, Ying Daia,c, Liang Haoa,b, Jinlong Zoua,b,*

a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's

Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Xuefu Road 74#, Nangang District, Harbin 150080, China.

b

Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion,

College of Heilongjiang Province, Heilongjiang University, Xuefu Road 74#, Nangang District, Harbin 150080, China.

c

School of Civil Engineering, Heilongjiang Institute of Technology, Hongqi Streent 999#, Daowai

District, Harbin 150050, China.

Corresponding author (s): * Jinlong Zou a

Xuefu Road 74#, Nangang District, Harbin, 150080, China.

Tel.: +86-451-86608549; Fax: +86-451-86608549 E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: To endow catalyst support with excellent stability and co-catalytic activity towards methanol oxidation reaction (MOR) is an effective way to strengthen the electrocatalytic activity of Pt-based catalysts. Tungsten phosphide/3D-corrugated porous carbon (WP/C) composite as Pt-support and co-catalyst for MOR is prepared via a synchronous synthesis method. Porous 3D-tufted structure and high surface area of WP/C with abundant oxygen-containing groups (such as C–O–C, C–O–H, or C–OH) can significantly improve the exposure of active sites, which enlarge the contact area with electrolyte and facilitate the mass transfer and absorption of methanol for promoting the MOR activity in acidic electrolyte. Pt-WP/C exhibits a considerably higher mass activity (1559.3 mA mgPt–1) for MOR than that of Pt/C (488.2 mA mgPt–1), owing to the special activity of Wδ+ and Pδ– sites for the decomposition reaction of water. With the introduction of W species, more available P species (passivated or not) are activated for further enhancing the co-catalytic activity of WP for MOR. Furthermore, the CO tolerance and durability of Pt-WP/C are also remarkable, which should benefit from the fast surface transport of adsorbed CO on different crystalline faces of WP and the extremely stable WP-C structure originating from the existence of P–P chains between the adjacent WP particles, respectively. The design of the porous structure and co-catalytic effect of this catalyst support (WP/C) provides a promising method to drastically enhance MOR activity.

KEYWORDS: Platinum catalyst support; Electro-catalytic activity; Co-catalytic action; Methanol oxidation reaction; Tungsten phosphide

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INTRODUCTION Direct methanol fuel cell (DMFC), which theoretically has the advantages including high energy density and convenient handling of electrolyte, is a promising power source for renewable and sustainable energy.1–4 Platinum (Pt)-loaded catalysts have been generally used as electrocatalysts in DMFC due to its high activity for both methanol oxidization in the anode and oxygen (O2) reduction in the cathode.2,5 Methanol is oxidized on the energetic Pt sites to generate carbon dioxide (CO2) and electrons, and the corresponding chemical reaction equations are shown below (Equations 1–3).2,4,5 Anode: CH3 OH + H2 O → CO2 + 6H+ + 6e

–

(1)

–

Cathode: 3/2O2 + 6H+ + 6e → 3H2 O

(2)

Overall: CH3 OH + 3/2O2 → 2H2 O + CO2

(3)

However, Pt-based catalysts exhibit low electrocatalytic stability in the anode resulting from CO poisoning of Pt active sites, which leads to the unstable energy density and conversion efficiency of DMFC.1,4 Furthermore, the exorbitant price of Pt restricts the industrialization of DMFC and the anode catalyst of DMFC is considered as the main restrictive factor for its popularization.6,7 Because of the irreplaceable function of Pt, the catalyst supports for Pt are in urgent need for improving the DMFC performance, which has already been acquired some achievements.5,8

To enhance the performance of Pt-based catalysts for methanol oxidation reaction (MOR), the investigation of novel supports with durability and co-catalytic effect is a promising approach.4,7–11 Carbon materials as catalyst (Pt, Pd, etc) supports have been used to enhance catalyst performance.7,9 Because the contact between C-based supports and catalytic components (metallic species) is so weak, the metal nanoparticles are easily agglomerated and 3



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stripped from the carbon surface, causing the decrease of active surface area and durability in long-term operation.9,10 Furthermore, under normal operating condition, carbon support and active components still suffer from electrochemical carbon corrosion and elements dissolution, respectively, which are major issues influencing DMFC performance.8,10 Huang et al. report that the synthesized Pt-nickel hydroxide (Ni(OH)2) is successfully loaded on graphene, which exhibits exceptional activity and durability towards MOR.12 The NiOH nanostructures with special defects in the as-prepared catalyst are very important for enhancing the dissociative adsorption of electrolytes and the oxidation of CO-like poisons on proximal Pt sites.12 Nonetheless, carbon-based supports only serve a supporting function and cannot provide the co-catalytic activity.12–14 Therefore, carbon-based composites with co-catalytic functionality are further investigated as novel catalyst supports, which can prevent or postpone the inactivation of the electrocatalysts and enhance the electrocatalytic performance of Pt-loaded electrocatalysts.8,10 Yan et al. synthesize a promising support, which

exhibits

a

positive

co-catalytic

effect

for

MOR.15

The

nanoscale

WN

particles/graphene has been synthesized as support to load Pt for preparing the MOR electro-catalyst.15 The close contact and strong interaction between WN and Pt enhance the activity and durability toward MOR.15 Despite the better performance, supports in this type inevitably possess inferior stability originating from the weak relationship between carbon phase and co-catalytic material.3,4 Thus, the synchronous synthesis of supports by embedding the co-catalysts in the carbon structure may be an effective method to enhance their self-stability and co-catalytic activity.4,16

Except for metal nanoparticles, metal oxides, composite materials and conducting polymers, transition metal phosphides (TMPs) as the interstitial alloys have been used as catalysts or co-catalysts for fuel cells, attributing to their excellent electrical conductivity and the specific P active sites.1,13,17–20 Especially, TMPs, such as molybdenum phosphide (MoP), germanium 4


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phosphide (GeP) and iron phosphide (FeP), exhibit comparable electrocatalytic activity and stability for hydrogen evolution reaction (HER), hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), which are similar to those of Pt-loaded catalysts.21–26 Xing et al. synthesize the HER catalyst of MoP nanoparticles with closely interconnected network structure, which exhibits high activity and low onset over-potential.27 For HER, the Mo species are used as hydride-acceptor center, while the basic P species are served as proton-acceptor center.27 The active P can also promote the generation of Mo-hydride for HER via electrochemical desorption.27,28 In consideration of the similar reaction steps (adsorption and decomposition of water, Equations S1–S10, Supporting Information, SI) between MOR and HER, the co-catalysis of TMPs towards MOR can be expected.4,26,29,30 In our previous work, a series of MoP/C composites as supports and co-catalysts are prepared, and the Pt-MoP/C electrocatalysts show higher MOR activity and durability than those of commercial Pt/C.31 Recently, tungsten phosphide (WP) has been used as an efficient catalyst for HER owing to its specific electronic structure and catalytic activity.32,33 Therefore, it may be feasible to use WP as a catalyst support and/or co-catalyst towards MOR, which may exhibit very interesting phenomenon.

In this study, WP/carbon (WP/C) composites as catalyst supports are prepared via a synchronous synthesis method, which are expected to possess co-catalytic functions for MOR. Citric acid as the carbon source and chelating agent is carbonized to form porous carbon, meanwhile the in situ formed nanoscale WP particles are firmly immobilized into the carbon phase. A thin oxide-film originating from passivation process is fitted onto the WP/C surface to prevent the over-oxidation and aggregation of WP species, which is verified by comparing with the contrast sample of nWP/C (n denotes non-passivation). Performance of WP/C and nWP/C as Pt supports (co-catalyst) for catalyzing the MOR has also been investigated. The particle-embedded structure of WP/C is conducive to the well-dispersion of 5



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Pt nanoparticles to provide more exposed active sites. The small-size WP crystals can provide plentiful P active sites, which are energetic for enhancing the co-catalytic functions for MOR. The Pt-WP/C catalyst is expected to exhibit a much higher mass activity and CO tolerance and superior long-term durability than those of commercial Pt/C.

EXPERIMENTAL SECTION Synthesis of WP/C and nWP/C composites Ammonium tungstate (H40N10O41W·xH2O, A.R.) and diammonium hydrogen phosphate ((NH4)2HPO4, A.R.) were together dissolved in deionized water. To promote the formation of WP nanoparticles, citric acid monohydrate (C6H8O7·H2O, A.R.) was dissolved into the above-obtained solution and the molar ratio of W: P: citric acid was set at 1: 1: 4. The mixture was heated at 90 °C with continuous stirring (water bath) for 12 h and then stirred at 80 °C to partly remove water to obtain a tawny transparent jelly, followed by thoroughly drying at 105 °C in an oven to evaporate the residual water. The dried jelly was heated with temperature-programmed carbonization process (N2, 50–60 mL min–1): (a) from 20 °C to 350 °C at a rate of 5 °C min–1; (b) from 350 °C to 900 °C (soaked for 2 h) at a rate of 1 °C min–1; and (c) natural cooling to around 20 °C. To avoid the deep oxidation of P species, the passivation of carbonized jelly was conducted in a mixed gas (volume ratio (O2/N2) of 1: 199) for 4 h. The passivated sample was grinded and marked as WP/C. The non-passivated sample (nWP/C) was obtained using the same route without passivated treatment.

Synthesis of Pt-WP/C and Pt-nWP/C catalysts A nominal 5 wt.% Pt was loaded onto the WP/C and nWP/C supports. 100.0 mg of WP/C support was suspended in aqueous chloroplatinic acid (H2PtCl6·6H2O, A.R.) solution and 6


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sonicated in water for 15 min to obtain a homogeneous slurry. The pH of homogeneous slurry (pH=10) was adjusted with 1 M NaOH solution. Excessive sodium borohydride (NaBH4, A.R.) solution was used as the reducing agent and the Pt-species was slowly reduced from the slurry at 25 °C with vigorous stirring. The slurry was constantly stirred for 3 h and then left for 12 h. The mixture solution was washed with deionized water (three times) and ethanol (two times) to remove Na+ and adsorbed water (centrifugation, 4000 rpm), respectively. The rinsed precipitate was finally desiccated at 50 °C to obtain the catalyst, which was marked as Pt-WP/C. By using the same method, the contrast catalyst marked as Pt-nWP/C was also synthesized. The Pt-loaded MoP/C (marked as Pt-MoP/C) sample with the best performance in our previous study was used as another contrast.31

Materials characterization and electrochemical analysis X-ray diffraction (XRD) analysis, N2 adsorption/desorption isotherms, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses were measured using the methods reported in our previous work to investigate the structural and surface characteristics of as-prepared supports and electrocatalysts.31 The working electrode with the as-synthesized Pt-WP/C electrocatalyst was prepared using the same method as reported previously.31 The working electrodes with commercial Pt/C (10 wt.%, HPT010, Hesen Electric Co., Ltd, Shanghai), Pt-nWP/C and Pt-MoP/C were used as the contrast electrodes. All of the set parameters and the electrochemical tests including electrochemically active surface area (ESAPt), cyclic voltammetry (CV), chronoamperometry (CA), electrochemical impedance spectroscopy (EIS) and CO stripping voltammetry, were referred to previous works.31,34–37 In this work, all 7



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of the reported potentials were versus the saturated calomel electrode (SCE, + 0.2415 V vs standard hydrogen electrode).

RESULTS AND DISCUSSION Composition and structure characterizations of WP/C and nWP/C composites As presented in Figure 1a, WP/C and Pt-WP/C show the typical XRD patterns of WP and the diffraction peaks at 21.1°, 28.7°, 31.0°, 42.9°, 43.2°, 44.6°, 46.5°, 56.7°, 68.8° and 73.6° correspond to the characteristic peaks of the (101), (002), (011), (202), (112), (211), (103), (020), (114) and (222) planes of WP (JCPDS, No. 29-1364), respectively.32,33 In each WP cell, four W atoms are existed on the opposing cell faces, while two W atoms and four P atoms are existed in the interior, which induce the P atoms to be linked with each other for forming the chains.38 Apparently, the peaks of WP/C are very strong, while nWP/C presents a relatively weak peak of WP phase, indicating that passivation is crucial for enhancing the crystallinity and stability of WP. As shown in Figure 1b, the (111), (200) and (220) planes of Pt (JCPDS, No. 65-2868) can be clearly observed at 39.8°, 46.2° and 67.5°, respectively. The differences in the crystallinity of Pt between Pt-WP/C and Pt-nWP/C are attributed to the WP-C structure with or without passivation. Note that O2-passivation can inhibit the deep oxidation and agglomeration of WP particles, which exerts positive influence on Pt nanoparticles dispersion. Moreover, some amorphous oxide species (such as W2OX, P2OX, etc) on Pt-nWP/C surface may induce the Pt(0) crystals to form platinum oxide (Pt2OX) and result in the agglomeration of Pt nanoparticles. Furthermore, no graphite peak can be detected at around 26° in Figure 1b, indicating that most of the skeleton in WP/C and nWP/C composites are amorphous carbon.

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N2 adsorption/desorption isotherms and pore size distributions of WP/C and nWP/C are presented in Figure S1 (Supporting Information, SI). The remarkable hysteresis loops can be observed, which indicates the mesoporous nature of WP/C and nWP/C composites. The typical type-IV isotherms can be observed in both of the curves. As shown in Table S1 (SI), WP/C possesses the larger BET surface area (SBET) of 392.39 m2 g–1 than that of nWP/C (220.77 m2 g–1). The similar pore volumes are obtained for WP/C (0.189 cm3 g–1) and nWP/C (0.156 cm3 g–1), while the pore size of WP/C (1.937 nm) is much smaller than that of nWP/C (2.834 nm), implying that passivation can effectively prevent the agglomeration and/or deep oxidation of WP particles to maintain the porous structure of WP/C. Porous structure (high SBET) and threadlike ducts (small pore size) of WP/C can supply abundant active sites for supporting Pt and provide abundant unhindered channels for mass and electron transfers to further facilitate the MOR.

The morphology features (SEM) of WP/C and nWP/C are presented in Figure 2. Numerous 3D-tufted bodies with corrugated and multihole structures are found on both WP/C and nWP/C surfaces. nWP/C obviously exhibits the aggregated structure with smooth surface, which is quite different from the morphology of WP/C. Without the passivation, the WP particles on nWP/C surface are easily aggregated to form the irregular lumps and the WP crystals on the surface/subsurface are deeply oxidized without the protective effect of thin oxidation film. In contrast, the porous 3D-tufted structure and high surface area of WP/C are extremely beneficial for further exposing the active edge sites to enlarge the contact area (chance) with methanol, which should facilitate the MOR activity.39 XPS measurements are conducted (Figure 3a) to research the surface components of Pt-WP/C and nWP/C. In the spectra of the two electrocatalysts, C 1s and O 1s peaks can be distinctly observed at 9



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around 284.6 and 532.2 eV, respectively. The W 4f and P 2p peaks are located at around 36.3 and 133.5 eV, respectively. The presence of Pt 4f peaks (around 71.2 eV) suggests that Pt species have been loaded on the surfaces of WP/C and nWP/C supports, in conformity with the results of XRD (Figure 1b). Moreover, the surface components of Pt-WP/C and Pt-nWP/C are listed in Table 1, which suggests the percentage content of each element on the catalysts surface. The contents of Pt Pt-WP/C (43.45 wt.%) and Pt-nWP/C (33.03 wt.%) are far higher than the theoretical value (5 implying that nanoscale Pt particles are homogeneously distributed on support surface and have covered the based species (WP and C). However, the low Pt content on Pt-nWP/C suggests that Pt atoms on the nWP/C surface are badly assembled to form the Pt nanoparticles (or clusters). The content (high exposure) of Pt on Pt-WP/C should provide more effective active sites for MOR.

The Pt 4f spectra of Pt-WP/C (Figure 3b), Pt-nWP/C (Figure S2a, SI) and commercial Pt/C (Figure S3, SI) are decomposed to three components, assigning to the Pt in different valence states (three pairs of doublets).40,41 As listed in Table S2 (SI), the Pt-WP/C and Pt-nWP/C possess higher proportions of Pt(0) than that of commercial Pt/C, indicating that both of as-prepared electrocatalysts can supply more metallic Pt sites for methanol electrooxidation. Compared with Pt/C, the binding energies of Pt(0) of Pt-WP/C and Pt-nWP/C catalysts reveal negative shifts of 0.4–0.9 eV, meaning that the specific electronic structure of WP changes the surface feature of Pt(0) originating from the intimate contact between Pt and WP. Moreover, the stable state of Pt(0) even in particular environment and the stable Pt-supports contacts are indispensable for enhancing the MOR activity and durability of Pt-WP/C and Pt-nWP/C.

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Figure 3c and Figure S2b (SI) show the W 4f binding energy curves of Pt-WP/C and Pt-nWP/C, respectively. The four components including WC, WO2, W 4f5/2 and W 4f7/2 can be derived by decomposing W 4f peaks. The W 4f5/2 peak at around 36.4 eV corresponds to the Wδ+ in WP, while the W 4f7/2 peak at around 38.5 eV corresponds to the W6+ species (such as WO3).39,42,43 Because WP/C and nWP/C samples have undergone O2 passivation and over-oxidation, respectively, the existence of the peaks corresponding to tungsten oxide in the spectra is not surprising. The formed WC species should play a certain role in enhancing the adhesion between WP and carbon skeleton.38 However, the diffraction peaks of WC cannot be found in Figure 1a, suggesting that the contents of WC species are infinitesimal in WP/C and nWP/C composites. Except tungsten oxide and WC, other kinds of W species cannot be found in the curves, suggesting that most of W is combined with P to generate WP. Compared with Pt-WP/C, the Wδ+ peak of Pt-nWP/C has a positive shift, attributing to that the electron cloud density of W atom in WP is reduced owing to the higher degree of W oxidation (over-oxidation).

As shown in P 2p spectra (Figure 3d and Figure S2c, SI), the P peaks in Pt-WP/C and Pt-nWP/C curves can be decomposed into two symmetrical bands originating from the spin orbital splitting, which correspond to Pδ– and P5+ species.39,42 The typical peak of P5+ species (P2O5) can be observed around 133.4 eV for both of Pt-WP/C and Pt-nWP/C, indicating that phosphorus oxides originating from the oxidation process (O2 passivation or over-oxidation) inevitably exist on the composite surface. However, considering the high crystallinity of WP species in WP/C (Figure 1a), the passivation (oxidization) of WP only happens in the shallow surface, not in the subsurface or The peak at around 129.7 eV confirms the presence of Pδ– species in WP. As indicated by the presence of Wδ+ and Pδ– species, WP is successfully synthesized in the carbon skeleton. 11



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As shown in Figure 3e and Figure S2d (SI), the deconvolution of O 1s spectra can be decomposed into four peaks, which correspond to the physically absorbed O or carbonates (around 530.4 eV), O–P (around 532.1 eV), C–O–C/C–O–H/C–OH (around 533.6 eV) and W–O (around 535.5 eV).42 The oxygen-containing groups (such as C–O–C, C–O–H, or C–OH) can dramatically enhance the hydrophilicity of Pt-WP/C and Pt-nWP/C catalysts, which is conducive to the mass transfer and methanol absorption for promoting the MOR activity in acidic electrolyte. The presence of O–W and O–P bonds in the O 1s peaks means that a slight trace of tungsten oxides (WO2 and WO3) and phosphorus oxides (P2O5) are existed in shallow surface of Pt-WP/C and Pt-nWP/C, consistent with the W 4f and P 2p analyses. As shown in Figure 3f and Figure S2e (SI), three components at around 284.6, 286.2 and 288.1 eV in the C 1s spectra correspond to the C in graphite, C–W and C–O, respectively.43 Formation of graphitic carbon in WP/C and nWP/C supports is important for enhancing the electrical conductivity, which indirectly improves the MOR activity.

Because the electrons can be transferred from W atom to P atom, W atom (Wδ+) should have some positive charge while P atom (Pδ–) should have few negative charge in WP.39,42 Because of the introduction of electronic defects, the W species may change the electron density of WP by comparing with W-P elementary substance alloy, which effectively enhances the cohesion between WP and Pt.32,33 Due to the passivation or oxidization, there are plentiful –OHads and –COOHads on the surface of WP/C and nWP/C, meaning that their surfaces may contain many negative charges. The negatively charged surfaces can induce repulsive interactions between Pt and WP, which are conducive to the well distribution of Pt nanoparticles on the supports. Because of the presence of positive charges on carbon surface, the charges should be easily 12


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transferred from WP to C skeleton to form a reproducible conductivity, which plays a decisive role in enhancing the methanol electrooxidation activity of Pt-WP/C and Pt-nWP/C.

Performance of different electrocatalysts CV curves (1.0 M H2SO4 solution) of Pt-WP/C, Pt-nWP/C and Pt/C electrocatalysts are conducted to obtain the number of useful electrocatalytic sites towards MOR (Figure 4a).4,34,44 Pt-WP/C catalyst exhibits much higher electrochemically active surface area (ESAPt) of 113.81 m2 g–1Pt than those of Pt-nWP/C (71.29 m2 g–1Pt) and Pt/C (58.28 m2 g–1Pt), indicating that Pt-WP/C has more available active sites for MOR. The porous 3D-tufted structure of WP/C, which can provide much more surface to load Pt and avoid the aggregation of active Pt nanoparticles, is greatly different from that of MoP/C (spheroidal structure, ESAPt of 103.90 m2 g–1Pt).31 WP/C as a co-catalyst can provide more catalytic active-area and promising conductivity, which can increase the contact chances between Pt-WP-C and electrons to enhance the consumption of electrons. The high ESAPt of Pt-WP/C means that more efficient and faster methanol oxidation is occurred on Pt surface, which is essential for enhancing catalytic performance of MOR.

CV tests of Pt-WP/C, Pt-nWP/C and Pt/C are conducted in methanolic acidic medium (Figure 4b Table 2). Generally, the methanol oxidation current peak of the electrocatalysts is located at around 0.7 V, while the peak corresponding to oxidization of the residual carbon species (intermediate products of COads and CHOads) is located at around 0.4 V.4,45 As presented in Figure 4b, Pt-WP/C shows a fantastic mass current density of 1559.3 mA mgPt–1 for MOR, which is far higher than of Pt-nWP/C (801.1 mA mgPt–1), Pt-MoP/C (680.7 mA mgPt–1) and Pt/C (488.2 mA mgPt–1).31 Such superior activity of Pt-WP/C may be related to the higher active surface area along with the faster 13



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transfer electrons from Pt to WP/C and the more active sites originating from the particular porous 3D-tufted structure. Both Pt-WP/C and Pt-nWP/C show higher electrocatalytic activity than that of Pt-MoP/C, implying that the co-catalytic effects of WP species are far stronger than that of MoP species. It should be noted that the quantity of active P sites is the main factor for influencing the co-catalytic activity toward MOR. With the introduction of W species, more P active sites are exposed on WP/C than that of MoP/C, which directly contributes to the high co-catalytic activity MOR. Furthermore, the intrinsic co-catalytic performance of WP/C is essentially associated with synergisms between the specific WP-C nanostructure and idiosyncratic electronic structure.39

In the forward scan, Pt-WP/C exhibits a lower onset potential and a higher peak potential than those of Pt/C, suggesting that Pt-WP/C can provide much more superior methanol electrochemical oxidation activity. The better performance of Pt-WP/C including high mass activity and low onset potential is ascribed to the uniform distribution of WP nanoparticles and more active Pt sites (high ESAPt). As reported previously, the ratio (If/Ib) of the methanol oxidation current peak (If) to the backward one (Ib) can preliminarily estimate the tolerance to the poisons including COads and CHOads.45–47 The If/Ib ratios of as-synthesized electrocatalysts and Pt/C are shown in Table 2, which exhibits that the ratios of Pt-WP/C and Pt-nWP/C are considerably larger by comparing with commercial Pt/C. It suggests that the WP/C structure can promote the fast mass transfer and adsorption of poisoning species to enhance the removal rate of CO-like intermediates in the forward scan, which is beneficial to liberate more Pt surface active sites. The tolerance to CO-like intermediates will further be verified by CO stripping experiments.

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CA tests are implemented to evaluate the durability of as-synthesized electrocatalysts and commercial Pt/C (Figure 4c). Mass activities of all of the electrocatalysts decline rapidly in the first 200 sec and then remain around a relatively constant value as time increases. After 3600 sec, the retained mass activities for Pt-WP/C, Pt-nWP/C and Pt/C electrocatalysts are 164.5 (decline of 89.5 %), 58.0 (decline of 92.8 %) and 18.6 (decline of 96.2 %) mA mgPt–1, respectively. As reported previously, Pt-MoP/C with a retained mass activity of 50.2 mA mgPt–1 (decline of 92.6 %) shows a worse stability than that of Pt-WP/C.31 Pt-WP/C presents the best durability (the lowest decline) for methanol electro-oxidation, which is mainly ascribed to the more efficient charge transfer and the stronger interactions among Pt nanoparticles, WP and C skeleton. WP nanoparticles are firmly embedded into the C skeleton for forming the WP-C structure, which can provide much more contact surface between WP and C, and form a very stable triple junction structure (Pt-WP-C). The specific triple junction structure means that Pt nanoparticles are in close contact with WP/C support for enhancing the electron transfer and MOR activity.48

The impedances of the working electrodes with as-synthesized electrocatalysts and Pt/C are using EIS measurements and the Nyquist curves are shown in Figure 4d. As shown in Table S3 the charge transfer resistances (Rct) and solution resistances (Rs) are calculated from the fits (an equivalent circuit diagram is shown in the inset figure) of the measured data.34,37 Rct corresponding the arc in the curve reflects the electro-catalytic activity towards MOR. Rct of Pt-WP/C is 12.23 Ω, which is slight larger than that of Pt/C (10.21 Ω), indicating that the promising electrical of WP/C should greatly contribute to the activity and durability for MOR. Although Pt-nWP/C electrode possesses a higher Rct (60.29 Ω) than those of Pt-MoP/C (17.87 Ω) and Pt/C, it exhibits a better mass activity for MOR. The Rs of Pt-WP/C and Pt-nWP/C are slightly superior to those of 15



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Pt-MoP/C and Pt/C. The reasons for the promising MOR performances of Pt-WP/C and Pt-nWP/C can be summarized and listed as below: (1) both the morphology of WP/C and the intrinsic property of WP should contribute the high co-catalytic activity for MOR; and (2) the large contact area (porous 3D-tufted structure and high surface area) with methanol can shorten the mass diffusion paths to accelerate the electronic transfer.39

The CO poisoning of Pt sites is a main restriction factor for MOR. To identify the CO tolerance of Pt-WP/C and Pt-nWP/C electrocatalysts, CO stripping tests are implemented (Figure 5).4,44 Compared with Pt/C (around 0.54 V), Pt-WP/C (around 0.50 V) and Pt-nWP/C (around 0.48 V) exhibit smaller onset potentials for CO oxidation. Pt-nWP/C shows the lowest onset and peak potentials for CO oxidation, while Pt-WP/C possesses the maximum peak area of CO oxidation, consistent with the results of CV (If/Ib) and ESAPt. It suggests that the CO oxidation capacities of Pt-WP/C and Pt-nWP/C are more efficient than that of Pt/C. The preferable performance for CO tolerance are ascribed to the fast CO adsorption on the WP species and oxygen-containing groups of WP/C, which is also associated with the fast surface transport of COads on different crystalline faces of WP.49 Note that passivation may significantly affect the state and exposure of the crystalline faces of WP crystals in WP/C or nWP/C. The dispersion and activity of Pt nanoparticles are also important factors for influencing the CO tolerance.

Stability of long-term operation with as-prepared catalysts The continued CV tests are performed using 1000 repeated potential cycles to further evaluate the durability of Pt-WP/C, Pt-nWP/C, Pt-MoP/C and commercial Pt/C (Figure 6). After 1000 cycles, 16


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residual (percentage) activities for Pt-WP/C, Pt-nWP/C, Pt-MoP/C and Pt/C are 62.2, 58.5, 51.7 and 44.1 %, respectively. Hence, Pt-WP/C shows the best durability for MOR and the remained mass activity (975.8 mA mgPt–1) is still superior to those of other catalysts. The significantly enhanced durability can be owed to the well dispersion of Pt nanoparticles on WP/C support and the intimate contacts between Pt and WP/C. Furthermore, the samples (Pt-WP/C and Pt-nWP/C) with WP exhibit higher durability than that of Pt-MoP/C and Pt/C attributing to the special structure of Pt-nWP/C shows the fastest decline of MOR activity in the first 200 cycles, which may be to the unstable WP-C structure without passivation. The thin oxide-film on WP/C surface from passivation plays a positive role in enhancing the durability of Pt-WP/C, while the excessive (gradual) oxidization of WP may reduce the exposed active sites of nWP/C to restrain the activity. Otherwise, the excellent catalytic performance of as-prepared catalysts can also be as follows: (1) WP/C guarantees the close interface contacts and electrical connections between WP and C, which facilitates the electrons flowing from Pt to WP/C; (2) the porous structure of WP/C is beneficial to the smooth diffusion of methanol and CO-like poisons; (3) the presence of oxygen-containing groups in Pt-WP/C can improve the adsorption strength of methanol and poisons, and avoid the block of active sites and diffusion pathways; (4) the P–P chains originating from the linked P atoms enhance the stability of WP/C structure.38,42

Detailed structure of the electrocatalysts and MOR mechanism TEM images and energy dispersive X-ray diffraction (EDAX) patterns of Pt-WP/C and Pt-nWP/C taken to investigate their microstructures and morphologies (Figure 7). As presented in Figure 7a Figure 7d, the nanoscale Pt particles with relatively homogeneous sizes are closely adhered onto the WP/C (nWP/C) skeleton. The Pt particles with smaller sizes are well dispersed on Pt-WP/C, while 17



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bigger Pt particles are found on Pt-nWP/C, implying that more Pt active sites of Pt-WP/C should be exposed. The measured data are fitted with a Gauss function and presented in the inset figures of Figure 7a and Figure 7d, which show that the average sizes of Pt nanoparticles for Pt-WP/C and Pt-nWP/C are approximately 1.93 and 2.48 nm, respectively. In the high-resolution TEM (HRTEM) images (Figure 7b and Figure 7e), the average sizes of WP nanoparticles for Pt-WP/C and are approximately 6.7 and 12.1 nm, respectively. The lattice fringes with d-spacing of 0.311 and 0.288 nm correspond to the (002) and (011) crystal planes of WP, respectively, suggesting that the WP particles with high crystallinity are successfully synthesized.38,42 Meanwhile, the Pt with the planes are tightly contacted with WP particles and/or carbon skeleton. As shown in Figure 7c and Figure 7f, the lattice spacing of 0.226 and 0.196 nm are ascribed to the (111) and (200) planes of Pt, respectively.40,50 The EDAX results are presented in Figure 7g and Figure 7h and the chemical compositions of Pt-WP/C and Pt-nWP/C are shown in the inset tables. The percentage contents of nanoparticles on the WP/C and nWP/C composites are approximately 4.8 and 5.1 wt.%, consistent with the theoretical value of 5.0 wt.%. The higher percentage contents of O-groups in Pt-nWP/C are attributed to the over-oxidation of nWP/C without passivation. As reported the presence of P-chains should enhance the acting force between the adjacent WP particles, which may wrap around the carbon skeleton to form extremely stable WP-C structure.38,42 After Pt deposition, the triple junction structure (Pt-WP-C) is effective to promote the charge transfer and durability and activity of the electrocatalysts for MOR.48

Notably, the electrocatalytic activity towards MOR is dramatically improved by using the WP/C as the support and co-catalyst, which is mainly attributed to the presence of special Wδ+ and Pδ– active sites. The process and mechanism of methanol electrooxidation by Pt-WP/C and Pt-nWP/C 18


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are deduced in Figure 8. The methanol molecules are easily adsorbed onto the support surface by OH/COOH/C-O-C groups and then oxidized on Pt active sites, which is more efficient and durable than the direct methanol adsorption and oxidation on Pt surface. For WP species, the pendant base P is close to the metal center of W, meaning that both of them may be the active sites for the decomposition reaction of water (the main and limiting steps in MOR).17 A proton of water is absorbed by Pδ– and then the remaining –OH is adsorbed on Wδ+ surface for forming the W-OH transition state.26,30 The W-OH is extremely unstable and possesses high activity for oxidizing PtCO species, which contributes to the liberation and recovery of Pt active sites.4 Moreover, the intermediate products like CO and CHO originating from the incomplete methanol oxidation can be easily adsorbed by oxygen-containing groups rather than Pt surface, and then are oxidized on the active Pt sites, which efficiently inhibits the invalidity of Pt active sites. The synergistic effects among oxygen-containing groups, WP and Pt play important roles in facilitating MOR activity and durability. The co-catalysis process of WP is deduced in the Supporting Information and the relative equations are shown in Equations S1–S12 (SI).

CONCLUSION WP/C and nWP/C composites with 3D-corrugated porous structure and high self-stability are successfully obtained via a synchronous synthesis method, which are used as the supports and co-catalysts for methanol electrooxidation. Citric acid is used as carbon source and chelating agent, which is conducive to the formation of WP particles with high crystallinity and good dispersion to enhance the intimate contact between WP and carbon skeleton. The specific WP-C structure is energetic for enhancing the MOR activity and durability of Pt-WP/C catalysts. Pt-WP/C exhibits the high ESAPt of 113.81 m2 g–1Pt, fantastic mass activity of 19



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1559.3 mA mgPt–1, and promising electrocatalytic durability (decline of 37.8 %) and CO tolerance towards MOR, which are much better than those of Pt-MoP/C and commercial Pt/C owing to the integrated functionalities of WP/C support. The special Wδ+ and Pδ– active sites can substantially enhance the MOR activity originating from their special activity for the decomposition reaction of water. The unstable intermediates of W-OH can change the reaction mechanism of MOR and further enhance the fast recovery of active Pt sites, which play a decisive role in promoting the CO tolerance. This work affirms the important role of the transition metal phosphide for MOR, which provides more new entry points into the synthetic method for support and the mechanism of co-catalytic effects for MOR.

ASSOCIATED CONTENT Supporting Information Additional details are available, including the derivational process of MOR mechanism, textural properties of WP/C and nWP/C composites (Table S1), binding energies and surface components for Pt 4f regions of Pt-WP/C, Pt-nWP/C and commercial Pt/C (Table S2), fitting results of electrochemical impedance spectroscopy (EIS) for Pt-WP/C, Pt-nWP/C and commercial Pt/C (10 wt.%) catalysts (Table S3), pore structure characteristics for the WP/C and nWP/C composites (Figure S1), high resolution XPS of Pt 4f, W 4f, P 2p, C 1s and O 1s for Pt-nWP/C (Figure S2), and high resolution XPS of Pt 4f for commercial Pt/C (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author 20


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*Phone/Fax: (+86) 451 8660 8549; E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the support by National Natural Science Foundation of China (51578218, 51108162, 51210105014, 21473051), Natural Science Foundation of Heilongjiang Province (B201411, QC2015009), Postdoctoral Science Foundation of Heilongjiang Province (LBH-Q14137), Scientific and technological innovation talents of Harbin (2016RQQXJ119), and Excellent Young Teachers Fund of Heilongjiang University and Hundred Young Talents in Heilongjiang University.

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Page 28 of 38

Table 1. Surface components of Pt-WP/C and Pt-nWP/C based on XPS results. Sample

Type

Pt

W

P

O

C

Pt-WP/C

Atomic conc/%

7.02

3.16

3.68

13.27

72.87

Mass conc/%

43.45

18.43

3.61

6.74

27.77

Atomic conc/%

4.46

2.89

3.04

15.66

73.95

Mass conc/%

33.03

20.17

3.57

9.51

33.72

Pt-nWP/C

28


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Electrochemical metrics for methanol oxidation on the Pt-WP/C, Pt-nWP/C and commercial Pt/C (10 wt.%) catalysts. Onset potentials

Peak potentials

Mass activity

(mV)

(mV)

(mA mgPt–1)

Pt-WP/C

316

707

1559.3

1.12

Pt-nWP/C

454

681

801.1

1.93

10 wt.% Pt/C

395

652

488.2

0.72

Sample

29


If/Ib


a Intensity (a.u.)

WP

WP/C nWP/C 10

20

30

40

50

60

70

80

2θ (Degrees)

b ★

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

Pt WP C

Pt-WP/C

Pt-nWP/C

★

Pt/C 10

20

30

40

50

60

70

80

2θ (Degrees)

Figure 1. XRD patterns of as-synthesized supports (a) and Pt-loaded electrocatalysts (b).

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. SEM images of WP/C (a, b) and nWP/C (c, d) composites.

31



a

b C 1s O 1s

P 2p W 4f

Pt-nWP/C

Pt-WP/C

0

200

400

600

800

1000

Pt4+ 74.8, 78.1 eV

68

1200

Pt2+ 73.0, 75.9 eV

Pt0 71.7, 74..9 eV

Intensity (cps)

Intensity (cps)

Pt 4f

70

72

74

76

78

80

82

138

140

Binding Energy (eV)

Binding Energy (eV)

c

d W 4f5/2, Wδ+

P , P2O5

36.2 eV

133.4 eV

5+

W 4f7/2, 38.3 eV

W4+, WO2 W-C, WC 31.9 eV

30

34.1 eV

32

34

36

38

40

42

P , WP 129.7 eV

Intensity (cps)

Intensity (cps)

δ-

44

126

128

130

Binding Energy (eV)

132

134

136

Binding Energy (eV)

e

f P-O, P2O5 532.1 eV O-physically absorbed or cabonated 530.4 eV

C-O-C, C-O-H, C-OH 533.6 eV

Intensity (cps)

Intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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W-O 535.5 eV

526

528

530

532

534

536

538

540

C in graphite 284.6 eV

C-O-C, C-O-H, C-OH 288.1 eV

280

Binding Energy (eV)

C-W 286.2 eV

282

284

286

288

290

292

294

Binding Energy (eV)

Figure 3. XPS survey spectra of Pt-WP/C and Pt-nWP/C (a) and high resolution XPS of Pt 4f (b), W 4f (c), P 2p (d), O 1s (e) and C 1s (f) for Pt-WP/C. 32


Page 33 of 38

a

60

b

1500

Pt-WP/C Pt-nWP/C 10% Pt/C

Mass Activity (mA mg-1 ) Pt


40 20 0 -20


-40 -60

1250 1000 750 500 250 0 -250 -500

-0.2

0.0

0.2

0.4

0.6

0.8

-0.2

1.0

0.0

d


900 750

0.4

0.6

0.8

1.0

35


30 25

600

-Zim (Ω)

c

0.2

Potential(V) vs SCE

Potential (V) vs SCE


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


450

20 15

300

10

150

5

0

0 0

500

1000

1500

2000

2500

3000

3500

0

10

20

Time (s)

30

40

50

60

70

Zre (Ω)

Figure 4. Cyclic voltammetry curves (a, in 1.0 M H2SO4 solution), formal Cyclic voltammetry curves (b), Chronoamperometry curves (c) and Nyquist plots (d) of Pt-WP/C, Pt-nWP/C and commercial Pt/C catalysts in methanolic acidic medium.

33



-0.2 80 60

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Pt-WP/C

40 20

-1

Mass Activity (mA mgPt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

0 -20 -40 60

Pt-nWP/C

40 20 0 -20 40

10% Pt/C 20 0 -20

-0.2

Potential (V) vs SCE

Figure 5. CO stripping voltammograms for Pt-WP/C, Pt-nWP/C and Pt/C catalysts.

34


Cycle Number

200

400

600

800

a 1500

b

1200 0.9

cycle 0 cycle 200 cycle 400 cycle 600 cycle 800 cycle 1000

900 600 300

0.8

0.7

0 0.6 -300

200

400

800


1.0

Relative Activity


0

1000

600 400

1000 1.0

0.9

0.8

200 0.7 0 0.6

-200

0.0

0.2

0.4

0.6

0.8

0.5 -0.2

1.0

0.0

0.2

200

400

0.6

0.8

1.0

Cycle Number

Cycle Number 0

0.4

Potential(V) vs SCE

Potential(V) vs SCE

c

800

-400 -0.2

750

600


600

800

1000

750

d

1.0

0

200

400

600

800

1000 1.0

600

300 150

0.8 0.7 0.6

0

0.5

-150

Relative Activity

0.9



600

450

Relative Activity

Cycle Number 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

450

0.8


300 150

0.7 0.6 0.5

0

Relative Activity

Page 35 of 38

0.4 -150

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.2

Potential(V) vs SCE

0.0

0.2

0.4

0.6

0.8

1.0

Potential(V) vs SCE

Figure 6. Continued voltammetry cycles and relative activity of Pt-WP/C (a), Pt-nWP/C (b), Pt-MoP/C (c) and commercial Pt/C (d).

35



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Figure 7. TEM images of Pt-WP/C (a) and Pt-nWP/C (d), HRTEM images of Pt-WP/C (b, c) and Pt-nWP/C (e, f) and EDAX patterns of Pt-WP/C (g) and Pt-nWP/C (h).

36


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Page 37 of 38

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Figure 8. Schematic of methanol oxidation by Pt-WP/C and Pt-nWP/C catalysts.

37



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Page 38 of 38

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